Project Summary We recently reported that levels of midbrain dopamine (DA) autoreceptors are inversely related to self-reported impulsivity. This finding suggests that individuals with reduced autoregulatory control of DA release are susceptible to impulsivity because of a reduced ability to control the approach-related motivational drive provided by DA neurotransmission. However, self-report measures do not allow for a direct examination of the core processes of behavioral control that are thought to be weakened in impulsivity. At present, the relation between DA autoreceptors and specific processes underlying impulsivity is unknown, as impulsivity is a multi- faceted construct and behavioral measures of impulsivity are only modestly associated with self-report scales. To clarify the link between DA functioning and impulsivity, we propose to measure midbrain DA autoreceptor availability and behavioral and fMRI indices of impulsivity in a group of 28 healthy adults. Midbrain autoreceptor availability, as well as D2-like receptor availability in the striatum and cortex will be assessed with the high affinity PET D2/D3 ligand [18F]fallypride. All participants will complete a rewarded stop signal task (SST) and a temporal (delay) discounting (TD) paradigm in order to provide objective measures of behavioral control. The SST and TD paradigms capture two distinct aspects of impulsivity (stopping and reward decision- making), which engage distinct, albeit overlapping neural networks. By completing these tasks during fMRI, we will be able to test whether individual differences in midbrain DA autoreceptor levels influence the responsivity of brain areas involved in self-regulation (e.g., anterior cingulate, ventral striatum, right inferior frontal gyrus). We will also test whether D2-receptor availability in the target behavioral control regions is predictive of the degree of BOLD activation during task performance, thus providing an ability to examine mechanistic models of the link between individual differences in DA functioning and the engagement of areas involved in behavioral control. In the case of TD, we will assess the hypothesis that lowered autoreceptor control leads to greater BOLD responses in the nucleus accumbens for immediate rewards, leading to more impulsive choice. In order to bridge current theories of DA functioning and the cognitive science of stopping, we have developed a novel, reward version of the SST that assesses the extent to which go and stop reaction times are altered by reward context. This may prove critical for defining the specific nature of DA's impact on behavioral control. Such reward manipulations have not been widely examined in the SST paradigm, despite the fact that most real-life impulse control problems arise in the context of high motivation. One hundred additional participants (and the 28 scanned participants) will complete the rewarded SST and a battery of self-report and behavioral measures of impulsivity to further characterize this new paradigm. Overall, this project bridges cognitive and affective science and neuroscience, behavioral economics, neuropharmacology and personality research to provide a framework for understanding reward influences on behavioral control and self-regulation.